Urea is commonly used as a fertiliser, supplying nitrogen to plants. Many soils also require sulphur as a plant nutrient, so fertilisers containing both urea and elemental sulphur have been developed. Desirably the elemental sulphur needs to be present as small dispersed particles to allow its oxidation in the soil to the plant available sulphate ion.
U.S. Pat. No. 3,100,698 discloses a urea-sulphur fertiliser that is made by combining molten urea and molten sulphur and subjecting the mixed melt to a prilling process. The mixed melt can also be prepared by adding solid urea prills to molten sulphur, or by adding solid sulphur to molten urea.
Melting sulphur and melting urea can be an energy intensive and therefore costly process, and can require sizeable equipment. Additionally, if melting of urea is not done quickly and in a controlled manner (i.e. the temperature is controlled such that it does not significantly exceed the melting point of urea), there is a risk of urea degradation. In particular, there is a risk of biuret production. Biuret is a phytotoxin (a material that is toxic to plants) and can be formed when urea is heated. Biuret interferes with nitrogen metabolism and protein synthesis in plants. It is desirable to reduce the amount of biuret in fertilisers.
The present inventors have sought to provide an improved process for the preparation of urea-sulphur fertiliser which is desirably simpler and more energy efficient than known processes. Preferably the process will allow for rapid yet controlled melting of fertiliser constituents, thereby reducing the risk of impurity formation whilst enabling the size reduction of sulphur to yield finely dispersed sulphur particles in the final product.